For data transmission on a standard voice grade telephone line, a modem is employed at each end of the telephone line. The modem is generally manually connected to the telephone line or automatically connected to the telephone line by appropriate connecting circuits. These connecting circuits may place the modem in an answering mode or an originating mode, which circuits accordingly alter the internal characteristics of each of the two communicating modems so that the data may be transmitted therebetween without interference or distortion. In addition, the modems become connected to or disconnected from the telephone lines according to a predetermined answering and disconnect protocol.
The modem and the associated answering circuits require a power supply which is usually operative from an AC power outlet or a battery source. In many instances the AC power source is not conveniently available, or in the case of a battery source, the modem may be in a location where battery monitoring or replacement is inconvenient. Additionally, the power supply adds to the expense and size of the modem. Therefore, to dispense with the need for any separate modem power supply or access to a separate power source, it is advantageous to provide power for the operation of the modem, and particularly its associated answering circuits, from both the telephone line and its associated data equipment over the existing connecting signal leads.
A modem is disclosed in a first copending application of O. Leon Pierce, Mark C. Smith and R. Byron Driver, entitled Line Powered Modem, Ser. No. 203,108, now U.S. Pat. No. 4,395,590 filed on Nov. 3, 1980 and assigned to the same assignee as this invention, in which the entire operating power for the modem circuits is derived from the available telephone line power.
In a second copending application of O. Leon Pierce, entitled Switchable Line Powered Modem, Ser. No. 203,111, filed on Nov. 3, 1980 and assigned to the same assignee as this invention, a modem is provided which derives its entire operating power from the available telephone line power in a manner similar to that described in the aforesaid copending application. Furthermore, the switchable line powered modem is selectively operable in an originate or answer mode for data transmission and reception in either of two operating frequency bands. The modem includes an interface and power source for coupling both signals from the telephone line to the modem for demodulation and signals from the modem to the telephone line for transmission.